1. Field of the Invention
The present invention relates generally to the field of disc drive data storage devices, and more particularly, but not by way of limitation, to a method for optimizing the operation of a disc drive actuator during a seek.
2. Discussion
Modern hard disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks by an array of transducers ("heads") mounted to a radial actuator for movement of the heads relative to the discs.
Typically, such radial actuators employ a voice coil motor to position the heads with respect to the disc surfaces. The heads are mounted via flexures at the ends of a plurality of arms which project radially outward from a substantially cylindrical actuator body. The actuator body pivots about a shaft mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs. The pivot shaft is parallel with the axis of rotation of the spindle motor and the discs, so that the heads move in a plane parallel with the surfaces of the discs.
The actuator voice coil motor includes a coil mounted on the side of the actuator body opposite the head arms so as to be immersed in the magnetic field of an array of permanent magnets. When controlled current is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the magnets and causes the coil to move relative to the magnets in accordance with the well-known Lorentz relationship. As the coil moves, the actuator body pivots about the pivot shaft and the heads are moved across the disc surfaces.
Typically, the heads are supported over the discs by actuator slider assemblies which include air-bearing surfaces designed to interact with a thin layer of moving air generated by the rotation of the discs, so that the heads are said to "fly" over the disc surfaces. Generally, the heads write data to a selected data track on the disc surface by selectively magnetizing portions of the data track through the application of a time-varying write current to the head. In order to subsequently read back the data stored on the data track, the head detects flux transitions in the magnetic fields of the data track and converts these to a signal which is decoded by read channel circuitry of the disc drive.
A closed-loop servo system is used to control the position of the heads with respect to the disc surfaces. More particularly, during a track following mode in which a head is caused to follow a selected data track, servo information is read which provides a position error signal indicative of the relative position error of the head to the track. The position error signal is used to generate a correction signal which is provided to a power amplifier, which passes current through the actuator coil to adjust the position of the head relative to the track.
The data tracks are divided into sectors so that a head can be moved to a selected sector on a selected data track by instructing the servo system to carry out a seek to that track. During a seek operation, the servo system receives the address of the destination track and generates control signals that cause the heads to initially accelerate and then subsequently decelerate as the head nears the destination track. It will be recognized that at some point towards the end of the deceleration of the head, the servo system will transition to a settle mode during which the head is settled onto the destination track and, thereafter, the servo system causes the head to follow the destination track in a fine control mode.
Typically, a velocity profile is used during a seek to control the movement of the head towards the destination track. The velocity profile determines the velocity the head should have at various distances from the destination track and, at each of a succession of tracks terminating with the destination track, the servo system provides a control signal to the power amplifier having a magnitude that is directly proportional to the difference between the profile velocity and the actual velocity of the head. The profile velocity is generally shaped with respect to the number of tracks covered in a seek and includes an initial acceleration portion, during which the heads accelerate from an initial radial velocity to some maximum velocity and a deceleration portion, during which the heads decelerate from the maximum velocity to a velocity of near zero at the destination track. In relatively long seeks, the heads will "coast" for a time at the maximum velocity after being fully accelerated towards the destination track, and this maximum velocity may be determined based on a variety of criteria, including for example, the maximum current that the power amplifier can reliably supply to the actuator coil.
It will be recognized that in the past, velocity profile have been designed so as to generally minimize the time required to move the head from an initial track to a selected destination track. Because the control signal provided to the power amplifier is proportional to the difference between the profile velocity and the actual velocity of the head, the head can be caused to rapidly accelerate towards the destination track during a seek by providing a velocity profile containing relatively large velocities at the beginning of the seek.
Generally, however, as the acceleration called for by the velocity profile is increased, a correspondingly greater amount of acoustic noise is generated by unmodeled mechanical resonances induced in the disc drive structure during the seek. Such noise has been found to be increasingly undesirable by disc drive users, and so efforts have been made to minimize the generation of acoustic noise during a seek, while at the same time minimizing the time required to perform the seek. A complicating factor in reducing the acoustic noise generated during a seek, however, results from the continuing trend in the disc drive industry to develop products with ever increasing areal densities (greater than 1 Gbit/in.sup.2) and decreasing access times (less than 10 ms). As this trend continues, greater constraints are being placed on the positional accuracy of servo systems and the effects of mechanical resonances on disc drive structures.
It will be recognized that the operation of the VCM in positioning the actuator with respect to the disc has been typically modeled as a single input, single output (SISO) system, with the input being the current provided to the actuator coil and the output being head position. The system is controlled with feedback derived from a Kalman filter state estimator. Such systems are typically 3rd order and may be controlled by way of the well-known Hamilton-Jacobi-Bellman (HJB) equations; see, for example the text by Bryson and Ho, "APPLIED OPTIMAL CONTROL", Hemisphere Publishing, 9th Edition, 1975, pp. 48-135.
Generally, the objective of a typical seek has been to move the head from the initial track to the destination track in a minimum amount of time (access time), with the only constraint being the amount of current available to drive the coil. Thus, a drawback to actuator control based upon the HJB approach is that the mechanical resonances excited during the seek are not modeled, yet these resonances can lengthen the time required to settle at the destination track as well generate undesirable acoustic noise.
Thus, one approach to minimizing the generation of acoustic noise during seeks has involved efforts to shape the velocity profile in such a manner as to reduce sharp transitions in the values of the profile, for example, see U.S. Pat. No. 5,475,545 entitled METHOD FOR REDUCING NOISE DURING SEEKS IN A HARD DISK DRIVE, issued Dec. 12, 1995 to Hampshire and McKenzie, assigned to the assignee of the present invention and incorporated herein by reference. Additionally, efforts have been made to shield the sound power generated during such seeks through the addition of acoustic damping material to disc drive assembly structures and housings.
More sophisticated attempts to reduce the generation of acoustic noise during seeks have involved designing a velocity profile input which as no residual Fourier components at poles of the dynamic structure; see, for example Katoh, "VIBRATIONLESS SEEKING CONTROL USING SLEW-RATE LIMIT FOR DISK DRIVES", 2nd International Conference on Motion and Vibration Control, Yokohama, Japan, September 1994. The drawback to this approach, however, is that a highly complex system (such as a disc drive structure) will have many poles and each pole will have a corresponding tolerance. Thus, the resulting control design will not be robust in terms of accounting for the real world tolerances present in high-volume disc drive manufacturing processes.
There is a need, therefore, for an improved approach to obtaining a velocity profile for a disc drive seek that optimizes seek-time constraints while at the same time minimizes the vibro-acoustic response of the dynamic structure of the disc drive.